A surface acoustic wave (SAW: Surface Acoustic Wave) device has been known as one of acoustic wave devices using acoustic waves. As a SAW device is small and light, and is able to obtain high attenuation, it is used in a variety of circuits that process radio signals in a frequency band of 45 MHz to 2 GHz in wireless devices such as mobile phone terminals. A variety of circuits include a transmission bandpass filter, a reception bandpass filter, a local filter, an antenna duplexer, an IF filter and an FM modulator, for example.
In recent years, with the development of high-performance and compact mobile phone terminals, it is demanded to improve frequency temperature characteristics and reduce the device size. To improve frequency temperature characteristics, there has been developed a technique that provides an oxide silicon film so as to cover a comb-shaped electrode formed on a piezoelectric substrate for example. In addition, it has been known that the optical absorption is increased when the value of X of SiOx is reduced in an oxide silicon, as disclosed in Japanese Patent Application Publication No. 11-6901 for example.
Frequency temperature characteristics may be improved by providing an oxide silicon film doped with an element so as to cover a comb-shaped electrode. This may cause a increase of the propagation loss of acoustic waves.